FR3084216A1 - DEVICE FOR EMULATING A BILAME, AND DEVICE FOR PROTECTING AN ELECTRIC LINE FROM OVERCURRENT - Google Patents

Abstract

The invention relates to an emulation device (EMU) of a bimetallic strip, the emulation device (EMU) comprising a current sensor (CC) capable of measuring a line current (IP) flowing through the device. emulation (EMU), the emulation device (EMU) being able to provide a value representative of a cumulative thermal state (Eth_n) in time t, called cumulative thermal state (Eth_n), by adding by recurrence a representative value d 'an initial thermal state (Eth_i), called initial thermal state (Eth_i), and a value representative of a current thermal state (Eth_on, Eth_off), called current thermal state (Eth_on, Eth_off), determined according to the line current (IP).

Description

DEVICE FOR EMULATING A BILAME, AND DEVICE FOR PROTECTING AN ELECTRIC LINE FROM OVERCURRENT

The invention relates to a device for thermal protection of an electric line. It applies in particular, but not limited to, the aeronautical field, and more specifically to the thermal protection of the cables which can supply an auxiliary power unit (used for example during the taxiing phases of an aircraft ).

Power lines are overheated mainly due to the flow of current. Overheating can create damage to the power line, for example a softening of the plastic surrounding the electric cable, or even smoke in the event of intense overheating. It is necessary to provide means for disconnecting the power lines in the event of overheating in order to protect yourself. The protection can extend to an electrical circuit connected to the power line, such as generators, power lines for the distribution of energy produced by generators and the loads supplied by the lines. The loads can be of several types: mainly resistive loads, for example implemented in avionics for heating or defrosting, and your mainly inductive loads for example formed by the windings of rotating machines.

It is for example possible to protect a line against an overcurrent by means of a fuse whose role is to disconnect the power line when the electric current flowing in the line exceeds a given intensity for a given time. Fuses can be time-delayed, in particular to allow the passage of large currents for short periods of time. This is particularly necessary when starting electrical machines, where a high current may be required. The fuse protection is not very precise and requires a change of the fuse when it has cut the current. The fuse is mainly used to compensate for serious short circuit faults.

In order to avoid having to change fuses, thermal circuit breakers can be used. Thermal circuit breakers are generally equipped with a bimetallic strip formed by a composite beam comprising two materials with different coefficients of thermal expansion. When an excessive current crosses the bimetallic strip, it deforms by the Joule effect mechanically causing the opening of the line. Compared to the fuse, the thermal circuit breaker has the advantage of not requiring its change in the event of tripping. Using the Joust effect to open the line tends to delay triggering.

However, the protection is, as with fuses, imprecise. In addition, bimetal circuit breakers have a fairly short service life. Typically, such a circuit breaker supports less than 4000 operations. In addition, a thermal circuit breaker cannot be reset remotely and therefore requires manual intervention.

The invention aims to overcome all or part of the problems mentioned above by proposing a circuit operating like a bimetallic strip and whose lifespan is increased.

An object of the invention is therefore a device for emulating a bimetallic strip, the emulation device comprising a current sensor capable of measuring a line current flowing through the emulation device, the emulation device being capable of providing a value representative of a cumulative thermal state in time t, called cumulative thermal state, by adding by recurrence a value representative of an initial thermal state, called initial thermal state, and a value representative of a current thermal state, called current thermal state, determined based on line current.

Advantageously, the emulation device comprises a square elevation operator configured to square a value representative of the line current from the current sensor.

According to a first embodiment, the emulation device comprises a programmable digital component configured to calculate the current thermal state by the relation:

Ip ² = 1

Eth_on ~~ Z 2 (1 6 f)

Αλ θύ E _than is the value of the current thermal state when the emulation device is supplied by the line current, where lp is the value representative of the line current, r a time constant, and / _tft a thermal current conventional of the emulated bimetal strip, and by the relation:

k'th.off -, 2 ^{X e τ} hh

Where E _th is the value of the current thermal state when the emulation device is not supplied by the line current, where lp is the value representative of the line current at the time when the emulation device n 'is more energized, r a time constant, and I _th a conventional thermal current of the emulated bimetallic strip.

Advantageously, the emulation device comprises a counter configured to determine the time since the moment when the emulation device is no longer supplied.

Advantageously, the programmable digital component is of the FPGA type.

According to a second embodiment, the value representative of the line current is an image voltage of the line current, the emulation device comprising at least one series RC circuit, and comprising at least one capacitor at the terminals of which the state is measured. cumulative thermal, the series RC circuit being connected to the square elevation operator, I initial thermal state being determined by the initial state of charge of the capacitor at the end of a charge or discharge cycle, the state current thermal being determined by I evolution of the state of charge of the condenser.

Advantageously, the square elevation operator comprises: a comparator, configured to compare the image voltage with a ramp signal, and to reinitialize the periodic ramp signal as soon as it is equal to the image voltage, thus generating a signal ramp with variable period having, for each variable period, a maximum amplitude equal to the image voltage at the same time;

an integrator, connected at the output of the comparator, and configured to integrate each ramp of the ramp signal with variable period over a period equal to the variable period, thus generating an integrated ramp signal, and for supplying the RC circuit with the integrated ramp signal .

Advantageously, the emulation device comprises a source of direct current capable of supplying a ramp capacitor with a continuous ramp current, the emulation device being able to reset the ramp signal by a controlled discharge from the ramp capacitor.

Advantageously, the emulation device comprises an envelope detection circuit, arranged between the squaring operator and the series RC circuit.

Advantageously, the emulation device comprises:

a device for comparing the image voltage with a reference voltage, the comparison device being able to supply a difference signal, representing the difference between the image voltage and the reference voltage; and a first voltage divider, coupled to a first switch controlled by the difference signal, the first voltage divider being configured, when the difference signal is positive, to divide the image voltage by a predetermined reduction coefficient.

Advantageously, the emulation device comprises a second voltage divider, coupled to a second switch controlled by the difference signal, the second voltage divider being configured, when the difference signal is negative, to divide the voltage of the signal supplied to the RC circuit series by the square of the predetermined reduction coefficient.

The invention also relates to a device for protecting an electric line against an overcurrent of the line current, the protection device comprising:

- an aforementioned emulation device;

a state comparator, configured to compare the cumulative thermal state with a target value;

- a switching device configured to disconnect the power line if the cumulative thermal state exceeds the target value.

Other characteristics, details and advantages of the invention will emerge on reading the description made with reference to the accompanying drawings given by way of example and which represent, respectively:

FIG. 1, an illustration of the selective levels associated with the thermal protection of a 25 Aeff line, for an aeronautical application;

- Figure 2, a block representation of the protection device according to the invention;

Figure 3, a block representation of the emulation device according to an embodiment called "analog";

Figure 4, a representation of the different analog components of the emulation device according to the invention;

The invention aims to emulate a bimetallic strip, using an emulation device, which reproduces the behavior of a bimetallic strip as faithfully as possible. The bimetal strip to be emulated is characterized by a trigger curve (also called trip curve). The tripping curve illustrates, for a given time, the maximum permanent current that the bimetallic strip can support, before opening, for example to trigger the protection of a line to be protected. By permanent maximum current is meant either a direct current or an alternating current. In the case of alternating current, the value of the maximum permanent current corresponds to the rms value.

Figure 1 illustrates your selective levels associated with the thermal protection of an AWG16 type copper line. Such a line has, in a standardized manner, a diameter of 1.29 millimeters. This type of line is commonly used in the aeronautical field, to circulate a current intended for a load, for example an electric motor. It is understood that the invention can be implemented whatever the section of the line and whatever its material as soon as it is able to conduct an electric current. The load absorbs an inrush current (also called "mrush current"). In FIG. 1, the inrush current of the load has a value for an infinite time estimated at 25 Aeff. For an infinite time, the tripping curve shows the conventional thermal current I _tb of the emulated bimetallic strip. This is the maximum current that the copper line can permanently carry without excessive heating.

The bimetal tripping curve must therefore be located above the curve representing the inrush current. The conventional thermal current / _tft must be located above the value of the inrush current (for an infinite time: 30 Aeff in the example in Figure 1). Protection redundancy can also be implemented with a fuse, in the event of bimetal failure. The fuse blowing curve is therefore located slightly above the bimetallic trip curve. In this case, the infinite value of the fuse blowing curve is 35 Aeff. Figure 1 also shows a curve called "no damage" (or "without damage"), and a curve "smoke" (or "smoke release"), specific to the line to be protected. It also represents the so-called “i ² t” model for a fuse, namely a modeling of the product of the square of the current over time.

The bimetal trigger curve can be modeled by the so-called bimetal relationship:

l —.................

Ip is the line current of the line to be protected, and for example measured by a current sensor.

τ is a predetermined time constant as a function of the inrush current imposed by the load, in particular in the initial phase of inrush current.

From the previous relation, we deduce that:

Ip ² (γ ~ 2 X 1 - 6 T = 1 ^l th '/

We define a thermal state E _thon , by the following relation:

Eth_on = x (1 - eT) (1)

If the thermal state E _thon is less than 1, we deduce from the two previous relationships that the measured line current I _P exceeds the tripping curve Conversely, if the thermal state _we are greater than 1, the measured current I _P is below the trigger curve.

In the absence of line current I _P , the thermal state is defined by the following relationship:

; 2 -t

Eth-Off ™ ^{xe T} (2)

In this case, in equation (2), I _P is the value representative of the line current at the moment when the emulation device EMU is no longer supplied with power, namely just before powering down.

We can then define a cumulative thermal state E _thn , which corresponds to the thermal state of the cumulative emulated bimetallic strip in time.

The cumulative thermal state E _thn is obtained by taking into account a current thermal state (which can be the thermal state E _tfl _ _on when the emulation device EMU is supplied, or the thermal state E _th when the device emulation emulation is not powered), and an initial thermal state

For example, during the initial powering up of an electric motor, a line current l _P having a particularly high value can be measured (so-called transient regime). Then, the value of the line current / _P can drop (so-called established speed). During the transient regime, the value of the cumulative thermal state E _th _ _n is obtained by adding the value of the initial thermal state E _thJ , which may possibly be zero, with the value of the current thermal state E _{tfl on} when the EMU emulation device is powered.

During the established regime, your value of the cumulative thermal state £ th._n is obtained by adding the value of the initial thermal state E _{tft t} , which is equal to the value of the cumulative thermal state E _thn at the end of the transient regime, with the value of the current thermal state E _tfl _ _on when the emulation device EMU is supplied. In this case, the value of the line current I _P is lower for the established regime than for the transient regime.

FIG. 2 illustrates a block representation of a protection device PRO integrating an emulation device EMU according to the invention. The LIG power line to be protected can be connected to a CIRC electrical circuit. The value of the cumulative thermal state E _t h _n is calculated by the emulation device EMU, from the line current I _P measured by the current sensor CC. A state comparator COMP2 compares the cumulative thermal state E _thn with a target value E _{th target} . The target value E _{th target} can be manually defined according to the LIG power line to be protected from a potential overcurrent. The protection device PRO also includes a switching device COM, which disconnects the line LIG if the state comparator COMP2 detects that the value of the cumulative thermal state E _thn is greater than the target value E _{th target} . The switching device COM can take the form of a switch controlled by the output of the state comparator COMP2. The switching device COM is on as long as the value of the cumulative thermal state E _{th n} is less than the target value E _thcible , and blocking in the opposite case.

In an exemplary implementation of the invention, the emulated bimetallic strip is subjected to an alternation of periods when it is supplied with current (periods of heating), with periods where it is not supplied with current (periods of cooling). We can then define by induction the value of the cumulative thermal state E _{th n} at the n ^th period.

The calculation of an exponential being particularly complex in numerical, one can carry out, for an implementation in a digital circuit, a calculation approximation for a time t small compared to the time constant τ:

-t t - e τ s r

And

-t t e-r si - T

Thus, for a n ^ems period, the value of the cumulative thermal state Eth_n ^es * determined by recurrence according to the following relation:

Ethn = + E _{th on n} = f- x (1 - TA) + ÎÙL · x ζΑ) * th. T 1 T ^ _ο // · _η-ι is the value of the thermal state at the end of the previous power-down period 7d. E _{th onn} is the value of the thermal state at the end of the current power-up period T ₂ . The periods of power off T _r and power on T ₂ may have different durations, or be equal for an AC input signal. / _{Pn i} is the value of the line current just before switching off, and / "la '* n value of the line current when switching on. By current value is meant the rms value of the current. In particular, for an AC input signal, the value of the line current Z _{Pn i} when switching off and the value of the line current l _Pn when switching on are equal.

According to a first embodiment, the aforementioned recurrence relation can be advantageously implemented in a programmable digital circuit, for example an FPGA circuit (for “FieldProgrammable Gate Array”, network of programmable gates), an ASIC circuit (for Application- Specific Integrated Circuit, application-specific integrated circuit), or even a microcontroller.

According to this embodiment, the emulation device EMU, implemented in a programmable digital circuit, includes a squaring operator. The operator is digitally programmed to square a value representative of the line current / _p from the DC current sensor, in accordance with relations (1) and (2). The operator can be programmed to raise the value representative of the line current l _p to a power significantly different from 2.

In the event that the emulation device is not powered (source de-energized), the programmable digital circuit should be equipped with a counter. Indeed, when the programmable digital circuit is not supplied, the counter makes it possible to record the duration of absence of line current. This time can then be used to determine the initial thermal state E _th

The invention can also, according to a second embodiment, be implemented in an analog circuit. For this, we use the relations (1) and (2) defined above. The proposed circuit implements analogically the squaring function of the line current I _p , as well as the functions (1 - eT) and (e ~), implemented respectively for the power-up periods and for the power-up periods. off the emulated bimetallic strip.

FIG. 3 illustrates the analog circuit according to the invention, in representation by functional blocks. In addition to this block representation, FIG. 4 shows in detail the components used, without limitation. The line current l _P , supplied by the current sensor CC, is transmitted to an intensity transformer Tl, in order to divide the value of the current by a predetermined factor Ns. This lowering of the intensity makes it possible to use in the analog circuit components capable of processing lower intensities. For example, the intensity transformer Tl can be configured to go from an intensity of the order of a hundred or even a thousand amperes, to an intensity of the order of Ampere, by applying the desired ratio to the number of primary turns relative to the number of secondary turns.

The negative half-waves of the lowered line current I _P are then rectified by a full-wave rectifier RA. The full-wave rectifier can be designed, for example, according to a diode bridge assembly, as illustrated in FIG. 4. The output of the full-wave rectifier RA is therefore the absolute value of the signal from the intensity transformer T1.

A resistor RES, illustrated in FIG. 4, is connected at the output of the full-wave rectifier RA. Thus, the value representative of the line current I _P is a voltage of the line current / _p , hereinafter called image voltage V _inc .

According to relations (1) and (2), a squaring of the value of the line current / _P must be carried out. In order to carry out such a squaring of the value of the line current I _P , it is possible to use a multiplier. which multiplies the value of the line current I _P by itself. However, analog multipliers are complex devices.

The calculation of the squaring can also be carried out by passing by the calculation of an integral. The integral is calculated on a ramp signal, produced by a ramp generator GR. A ramp signal is generated by a load of a ramp capacitor C _ramp , shown in FIG. 4. The ramp capacitor C _ramp is supplied by a continuous ramp current i _ramp , generated by a DC current source SCC. An operational amplifier AO1 mounted as a follower can be coupled to the ramp capacitor C _ramp and the direct current source SCC, in order to carry out an impedance adaptation with the next stage. A comparator COMF1 receives as input the image voltage Vi _nc and the ramp signal. The comparator COMF1 can take the form of an operational amplifier AO2 which determines the difference between the image voltage V _inc and the ramp signal, as illustrated in FIG. 4. The difference is compared to a zero reference voltage. From the characteristic relation of the capacitor, it is possible to establish, when the image voltage V _inc and the ramp signal have the same amplitude, the following relation:

tramp c 'ramp

Vine iramp (3)

At the instant when the image voltage V _inc and the ramp signal have the same amplitude, the comparator COMF1 controls the discharge of the ramp capacitor C _ramp . A transistor TRO, itself coupled to the comparator COMF1, can be placed in parallel with the ramp capacitor ^ ramp s in order to control the discharge of the ramp capacitor C _ramp . Once discharged, the ramp capacitor C _ramp recharges again, until the image voltage V _inc and the ramp signal have again the same amplitude.

The output of the comparator COMF1 is therefore a ramp signal with variable _ramp t period having, for each variable _ramp t period, a maximum amplitude equal to the image voltage V _{ir: c} at the same instant. The ramp signal with variable _ramp period t then passes through an INTG integrator. The integrator INTG, shown in detail in FIG. 4, is characterized by a resistor R _int and by a capacitor C _int on the non-inverting input, as well as by two resistors Rl _int and R2 _int on the inverting input. The INTG integrator determines an integrator output voltage ί (RZjnt; -s \ h-amp [ _R1 + çtramp

Vinjnt ~~ f A „---- XI tdt ^ intLrampRmt Jq if 2 (R ^ int, 4 λ ⁱ ramp ^t -ramp J ^ in_int X'X A ~ ^ώ mt ramp ⁿ int

Thus, from relation (3): Vinjnt ~ ^ iYétu · ² “^ 2 ^ p ²

Where ky and fc ₂ are constants of proportionality.

The square elevation operator CAR, consisting of the comparator COMP1 and the integrator INTG, thus makes it possible to obtain a square of the value representative of the line current I _P.

An envelope detection circuit CDE is coupled to the squaring operator CAR, in order to recover the envelope of the squared signal. Different types of envelope detection circuits can be used, in particular a peak detection circuit. Advantageously, the CDE envelope detection circuit is a sampler-blocker. It includes a switch and a capacitor C1. The switch is controlled by a sampling signal echJ2, as illustrated in Figure 4. As long as the switch is closed, the capacitor acts as a memory element vis-à-vis the signal from the integrator INTG . When the switch receives an opening pulse, the capacitor, isolated from the signal from the integrator INTG, restores the stored value. Advantageously, the sampling signal fixed by the comparator

COMP1. It transmits the opening pulse each time the image voltage V _inc and the ramp signal have the same amplitude. Thus, the sampler-blocker recovers, for each variable period t _ramp , the maximum of the ramp signal with variable period, with a constant output between each opening command. An envelope substantially equal to the square of the image voltage can thus be recovered.

The value of the cumulative thermal state E _t h _n is measured across the capacitor of a RC circuit CIRC_RC series. The series RC circuit CIRC_RC comprises a resistor Rm connected directly or indirectly to the envelope detection circuit CDE, and a capacitor Cm at the terminals of which the value of the cumulative thermal state E _{tfl n} is measured. The charge of the capacitor Cm emulates the behavior of a bimetallic strip during the heating phases. The series RC circuit Cl RC ... RC thus performs the function -i (1 - e *), described in relation (1). The discharge of the capacitor Cm emulates the behavior of a bimetallic strip during the cooling phases. The RC circuit CIRC__RC series thus performs the function (e ~), described in relation (2). It is to highlight that :

t ~ Rm x Cm

The time constant r is found in the relation of the bimetallic strip. The choice of the numerical values of the resistance Rm and of the capacitor Cm therefore determines the shape of the bimetallic trip curve, which should preferably be located above the inrush current curve imposed by the load.

The aforementioned emulation device is configured to work with a line current I _P having a high dynamic range. The value of the line current I _P can in particular pass from a nominal current value I _n , which corresponds to the maximum value of the inrush current (200 Aeff in I example of FIG. 1). at a value ten times smaller in steady state. Likewise, the value of the line current I _P can greatly exceed the value of the nominal current Z _n , for example when starting an electric motor. The value of the line current / _P can at times be ten times smaller than the nominal current value I _n , and at times be ten times greater. This important dynamic can generate a loss of precision for the determination of the cumulative thermal state E _{th n} .

Thus, for a line current I _P having a value greater than the value of the nominal current / „, the image voltage V _inc of the line current I _P is divided by a predetermined reduction coefficient k. For this, a first voltage divider DT1 is connected to the resistor RES. The first voltage divider DT1 can be of the resistive type. In FIG. 4, a first controlled switch TR1 is on when the image voltage V _inc is detected as being greater than a threshold voltage V _ref . The threshold voltage V _re y is determined as a function of the nominal current value I _n . In this case, a fraction of the signal from the resistor RES is taken across the resistor of smaller value. Detection of exceeding the threshold voltage V _re f. can be performed by an AO3 comparator. The comparator AO3 supplies a difference signal Div, representing the difference between the image voltage V _inc and the reference voltage Vref.

When the comparator AO3 does not detect that the threshold voltage V _{ref is} exceeded, the voltage from the envelope detector circuit CDE is divided by the coefficient k ² , and the image voltage V _inc of the line current I _P is not not by divided by the reduction coefficient k. For this, a second voltage divider DT2 is connected between the envelope detector circuit CDE and the series RC circuit CIRC_RC. A second controlled switch TR2 is on when the image voltage V _inc is detected as being lower than the threshold voltage V _re f, namely when the difference signal Div is negative.

Other embodiments can be envisaged, in order to take account of the great dynamics of the line current 1 _P. For example, a high threshold (for example much higher than the nominal current l _n ) and a low threshold (for example much lower than the nominal current / „). can be defined. In this case, the reduction coefficient k is only applied to the image voltage V _inc if the line current is greater than the high threshold, and the reduction coefficient k ² n is applied to the output of the envelope detector circuit CDE only if the line current is below the low threshold.

Claims (12)

1. Emulation device (EMU) for a bimetallic strip, the emulation device (EMU) comprising a current sensor (CC) capable of measuring a line current (/ _p ) flowing through the emulation device (EMU), the emulation device (EMU) being able to provide a value representative of a cumulative thermal state (E _t h_ _n ) in time t, called the cumulative thermal state (Eth „n), by adding by recurrence a representative value d 'an initial thermal state (E _îh j), called initial thermal state (E _th j), and a value representative of a current thermal state (E _îh _ _on , E _t h_ _O ff), called current thermal state (And _h _on, E _t h_ _O ff), determined according to the line current (/ _P ). 2. Emulation device (EMU) according to claim 1, comprising a squaring operator (CAR) configured to squaring a value representative of the line current (I _P ) from the current sensor (CC). 3. An emulation device according to claim 2, comprising a programmable digital component configured to calculate the current thermal state (E _E h_ _on , E _t h_ _O ff) by the relation: lp ² zl Ethon = ““ 2 X (1 “e T) Ah θύ E _thon is the value of the current thermal state when the emulation device (EMU) is supplied by the line current (I _P ), where 1 _P is the value representative of the line current (J _P ), τ a time constant, and I _th a conventional thermal current of the emulated bimetallic strip, and by the relation: h ^{2 E} th_off - --zxe? ha Where ^E th_off is the value of the current thermal state when the emulation device (EMU) is not supplied by the line current (/ _P ), where Z _P is the value representative of the line current (7 _P ) at the moment when the emulation device (EMU) is no longer supplied, τ a time constant, and I _th a conventional thermal current of the emulated bimetallic strip. 4. An emulation device according to claim 3, comprising a counter configured to determine the time (t) since I instant when the emulation device (EMU) is no longer supplied. 5. Emulation device according to one of claims 3 or 4, the programmable digital component being of the FPGA type. 6. An emulation device according to claim 2, the value representative of the line current (/ _P ) being an image voltage (Vine) of the line current (/ _P ), the emulation device (EMU) comprising at least one series RC circuit (CIRC_RC), and comprising at least one capacitor (Cm) at the terminals of which the cumulative thermal state is measured (E _t h_ _n ), the series RC circuit (CIRC_RC) being connected to the elevation operator at square (CAR), the initial thermal state (E _th j) being determined by the initial charge state of the capacitor (Cm) at the end of a charge or discharge cycle, the current thermal state (E _t h_ _on , E _t h_off) being determined by the evolution of the state of charge of the capacitor (Cm). 7. An emulation device according to claim 6, the squaring operator (CAR) comprising: - a comparator (COMF1), configured to compare the image voltage (Vine) with a ramp signal, and to reset the periodic ramp signal as soon as it is equal to the image voltage (Vine), thus generating a ramp signal with variable period (t _ramp ) having, for each variable period (t _ramp ), a maximum amplitude equal to the image voltage (Vine) at the same time; - an integrator (INTG), connected at the output of the comparator (COMP1), and configured to integrate each ramp of the ramp signal with variable period over a period equal to the variable period (t _ramp ), thus generating an integrated ramp signal, and to supply the RC circuit with the integrated ramp signal, 8. An emulation device according to claim 7, comprising a direct current source (SCC) capable of supplying a ramp capacitor (C _ramp ) with a continuous ramp current (i _ramp ). the emulation device (EMU) being able to reset the ramp signal by a controlled discharge of the ramp capacitor (C _rarnp ). 9. Emulation device according to one of claims 6 to 8, including an envelope detection circuit (CDE), arranged between the squaring operator (CAR) and the serial RC circuit (CIRC_RC). 10. Emulation device according to one of claims 6 to 9, including: - a comparison device (AO3) of the image voltage (Vine) with a reference voltage (Vref), the comparison device (AO3) being able to supply a difference signal (Div), representing the difference between the image voltage (Vine) and the reference voltage (Vref); and a first voltage divider (DT1), coupled to a first switch (TR1) controlled by the difference signal (Div), the first voltage divider (DT1) being configured, when the difference signal (Div) is positive, to divide the image voltage (Vine) by a predetermined reduction coefficient (k). 11. An emulation device according to claim 10, comprising a second voltage divider (DT2), coupled to a second switch (TR2) controlled by the difference signal (Div), the second voltage divider (DT2) being configured, when the difference signal (Div) 5 is negative, to divide the voltage of the signal supplied to the series RC circuit by the square of the predetermined reduction coefficient (k). 12. Protection device (PRO) of an electric line (LIG) against an overcurrent of the line current (/ _P ), characterized in that the 10 protection device (PRO) includes: - an emulation device (EMU) according to one of the preceding claims; a state comparator (COMP2), configured to compare the cumulative thermal state (Eth_ _n ) with a target value of $ ¹ (Efh_cibte) - a switching device (COM) configured to disconnect the power line (LIG) if the cumulative thermal state (Eth__n) exceeds the target value.